Rubbery isoolefin copolymer composition of improved cold resistance



Patented Nov. 8, 1949 RUBBERY ISOOLEFIN COPOLYMER COMPO- SITION OF IIWPROVED COLD RESISTANCE Winthrope 0. Smith, Westfield, N. .L, assignor to Standard Oil Development Company, a corporation of Delaware No Drawing. Application October 26, 1945, Serial No. 624,957

3 Claims.

physical characteristics.

Rubber articles intended for use where low temperatures are encountered as in automobiles and airplanes are required to remain flexible at temperatures as low as 40 F. Special vulcanizates of natural rubber with certain sulfur ratios and certain accelerators have shown good flexibility at subzero temperatures. Certain vulcanizates of synthetic rubbery polymers, particularly the vulcanized oleflnic copolymers of olefins with diolefins, have shown as good flexibility in comparison to natural rubber vulcanizates at moderately low subzero temperatures, but it has been found desirable to improve their low temperature flexibility still further to meet the stated requirements.

Improvement of low temperature flexibility of oleflnic copolymer vulcanizates has to be obtained by a distinctive manner of compounding. In general, the rubbery olefinic copolymers do not need the mastication required by natural and relatively hard and dry synthetic rubbers in processing, and materials among those most useful as softeners to facilitate the processing of such rubbers do not have a desired effect on the physical properties of cured olefinic copolymers. Some of these softeners interfere with vulcanizates of the olefinic copolymers, and others have an adverse or no substantial favorable efiect.

An object of this invention is to impart substantially improved cold resistance or low temperature flexibility to a vulcanizate of oleflnic copolymers by incorporating a suitable phosphate ester.

A more specific object of this invention is to provide for utilization of relatively high molecular weight alkyl phosphates having 16 or more carbon atoms per molecule in alkyl radicals, for conferring on vulcanized oleflnic copolymers of isobutylene with isoprene required flexibility at temperaturesas low as 40 F. and lower without deleterious eflects on other desirable physical properties of the vulcanizate.

The olefinic copolymers are polymeric materials synthesized by low temperature catalytic polymerization of an olefin, such as isobutylene, with a diolefln, such as isoprene. These copolymers have become known by the term butyl rubber." Methods of manufacturing and compounding these materials are given in the U. 5. Patent 2,356,128 of August 22, 1944, to R. M. Thomas and W. J. Sparks. Properties of these materials are further described in an article beginning on page 1282 of Ind. Eng. Chem, vol. 32, No. 11, October 1940.

Although procedures for the preparation of butyl rubber are described in the above-mentioned patent, a general method of preparation will be briefly outlined.

Butyl rubber is prepared by reacting a monoolefin, such as isobutylene, with a diolefln, such as isoprene, at a temperature below 0 C. in the presence of a Friedel-Crafts catalyst, such as aluminum chloride. The mono-olefin reactant is preferably used in a major proportion with respect to the diolefln. In a typical preparation, /2 to 20 parts of isoprene and 99% to parts of isobutylene are mixed with a solution of aluminum chloride dissolved in an organic solvent which forms no complex with the catalyst and is liquid at the reaction temperature. The reaction temperature is preferably in the range 50 C. to about 165 C. A useful solvent for the metal halide catalyst is an alkyl halide, such as methyl chloride or ethyl chloride.

The resulting oleflnic copolymer is a tough, elastic product resistant to oxidation and chemical attack. It is characterized by low unsaturation, e. g., an iodine number below 50. Copolymers of this type have been estimated to have average molecular weights above 15,000 and often between 30,000 and 80,000. Butyl rubber can be processed on conventional rubber machinery. It becomes soft and plastic at roll temperatures of the order of 80 to 100 C. Vulcanization of the copolymers is effected by heating with sulfur and zinc oxide, and with other sulfurization aids as described in the above-mentioned patent, which also shows that other compounding agents, including certain fillers and softeners, may be employed.

Now iii accordance with the present invention it is found that in orderto impart improved cold resistance to the vulcanized olefinic copolymer product a specific type of agent must be used.

Ester derivatives of phosphoric acid now found to be particularly eflective agents for increasing the cold resistance of butyl rubber or oleflnic copolymer vulcanizates are such alkyl phosphates as diand tri-octyl phosphates. In general, it appears the suitable alkyl phosphate should contain from 16 to 36 alkyl carbon atoms per molecule to arrive at satisfactory results. These alkyl phosphates may be obtained by esterifying phosphoric acid with higher alkanols having 8 to 12 carbon atoms per molecule, such as octanol, decanol, or dodecanol. Also, it is possible to use The data illustrates how the organic phosphates, with increased size of alkyl radicals, confer greatly improved cold resistance on the vulcanizates. It is to be noted that the dioctyl and trioctyl phosphates with 16 and 24 alkyl carbon atoms in the alkyl radicals, respectively, give a considerable greater improvement than the other phosphates and that their incorporation into the vulcanizates accomplishes the objects of this inmixtures of such higher alkanols or mixtures convention. The triphenyl phosphate which conmining an pp p Proportion of lower tains no alkyl carbon atoms actually degrades kanols with the higher alkanols to obtain the the product. Phosphates represented by tris ita l alkyl ph ph e Fr about 5 to 60 cresyl and tributyl phosphates, both of which parts by weight of these suitable esters based on h l th 16 alkyl carbon atoms per molee Weight of u polymer gum are to be cule, give no substantial improvement with reincorporated with 100 parts by weight of the gum spect, to freeze resistance. during P A further test determines the flexibility and In determinmg the cold freeze resistance resilience imparted to the copolymer by the addiquellties 0f the v ee t a method termed tion of the alkyl phosphates in terms of recovery The envelope freeze test" was used. This is a 2n and Speed f recovery after compress10n very severe test conducted on samples of the The formula used is as follows; vulcanizate stocks calendered on fabrics. In these tests a wide variety of addition agents were Copolymer parts- 100 compared. Zinc oxide do 5 Sulfur do 2 TEST PROCEDURE Tetramethyl thiuram disulfide do 1 A piece of the calendered fabric 4" x 6" is Mercaptobenzothiazole d0 0.5 folded in half, with the rubber or vulcanizate SRF Black (Gastex) d0- coating on the inside. With the folded edge of 30 Alkyl phosphate volumes 10 this resultant 4" x 3" specimen at the bottom, the two bottom corners are folded again into two Molded cylinders dlametel and isosceles triangles 1 X 1 x leaving a height were cured of each stock for minutes at portion of the original fold between them. The The test a Conant in a methanol envelopes are placed in the cold box at the specibath at 115mg an ASTM asphalt D fied temperature with a 650 g. weight over t trometer with a foot in diameter. Three folds. After five hours the envelopes are snapped Samples Were u a d the esu t a e d on open as rapidly as possible in the cold b x by each stock. The slugs were preconditioned by grasping the top open ends. Cracking through cycling once through 20% compression, then beto the fabric at the fold constitutes a failure. m ing compressed 15% of their original height, held Representative data on the vulcanizate formufor ten minutes and released, taking reco y lation and test results are as followszreadings at 3, 5, 10, 20, 40, and seconds.

Formulae A B o D E F Isobutyiene-Isoprene Copolymer 100 100 100 100 100 Zinc Oxide 5 5 5 5 5 5 Stearic Acid" l l l l 1 1 Tuads l 1 l 1 1 l Selenac i l l l l 1 l Suliun- 2 2 2 2 2 2 Wax 3 3 3 s 3 3 Clay 75 75 75 75 75 75 Triphenyl Phos hate 'lricresyl Phosp ate. Tributyl Phosphate. Trioctyl Dioctyl Phosphate Cure60at287 F. A B c D E I F TeusileElongation 2,410-130 1,770-810 2, 070-750 2,000- 2, 010410 2,090-810 Modulus atBOO-Shore 350- 45 so- 31 so 120- 34 so as Freeze resistanceEnveZope test I A I B o D l E F 20F OK Bad Crease. 0K OK 0K 0K. 40 F. Failed Failed Bad Crease. Bad Crease. 0K 0K. 50F i Failed Failed Sl.Crcase 0K. -60 F Bad Crease. SLCi-ease. 70F Failed i. Failed.

I 1 Tetramethyl thiuram disulflde. 1 Selenium tetraethyl dithiocarbamate. Slight crease.

The following data were obtained:

Time in Seconds... 7 a s 1c 20 4o 00 Recove inper oentz Con rol 30.5 39.2 49.0 58 66 73 'Dloctyl Phosphate 59.8 66 72.1 77.9 81.4 84.6 'lrioctyl Phosphateuu n 60.3 05.9 72.5 77 81 86 Thus the great increase in-recovery and recovery rate at low temperatures brought about by the use 0! dioctyl and higher alkyl phosphates .is demonstrated. This test is applicable to mechanical goods and innertubes whereas the envelope test can be applied to prooied goods.

Advantageously, I the coldre'sistant butyl rubber stocks improved by the-higher alkyl phosphates remain light in color, have little or no odor, and have a low heating loss, resulting in retention oi. desired freeze resistance properties 'over long periods of time. These suitable phosphates do not destroy the normal tack o! the butyl rubber, thus permitting normal building operations, and allowing good adhesion to labrics. Therewas no evidence of bleeding from the uncured or cured stocks containing these phosphates, thus showing good compatibility. The tensile strength of the stocks is less affected by these higher alkyl phosphates than by the other phosphates or by other additives previously used. Also,.they lower the modulus to a lesser degree than other additives.

The specially useful organic phosphates have accordingly been demonstrated to be esters of phosphoric acid in which hydrocarbon radicals contained at least 16 alkyl carbon atoms per molecule.

6 It is not intended that this invention be limited by the examples given for the purpose of illustration, since modifications thereof as set forth may be made without departing from the spirit and scope of the invention deflnedin the appended claims.

I claim:

1. A cold resistant vulcanizate of a synthetic rubber of 80 to 99% parts of isobutylene and /2 to 20 parts of a conjugated diolefin copolymerized at a temperature between -50 C. and 165 C. by "means of a Friedel-Crafts catalyst, the vulcanizate containing 100 parts by weight of said rubber and 15 parts by weight of an ester of phosphoric acid chosen from the class consisting of dioctyl phosphate and trioctyl phosphate.

2. A composition according to claim 1 in which the ester is dioctyl phosphate.

3. A composition according to claim 1 in which the ester is trioctyl phosphate.

WINTHROPE C. SMITH.

REFERENCES CITED The following references are of record in the file oi this patent:

UNITED STATES PATENTS 

